1. Field of the Invention
The invention relates to a steam turbine and, in particular, to a steam turbine with an improved cooling system for the casing.
2. Description of the Related Art
With reference to FIG. 3, a prior art stream turbine includes a rotor 100 extending along a longitudinal axis, and a casing 102 enclosing the rotor 100. A high pressure turbine portion 103, an intermediate pressure turbine portion 104 and a lower pressure turbine portion 105 are disposed within the single casing 102 around and along the rotor 100.
Provided within the casing 102 is a dummy ring 110 which separates the high and intermediate pressure turbine portions 103 and 104 and seals therebetween. The dummy ring 110 and the casing 101 define a space 118 therebetween. The space 118 is filled with steam so that the steam within the space 118 is held there. The steam within the space 118 is heated by thermal transfer from the high pressure and temperature steam supplied to the high pressure turbine portion 103 so that the portion of the casing 102 enclosing the space 118 is also heated. This results in the thermal deformation of the casing 102.
The invention is directed to solve the above mentioned prior art problems, and the objective of the invention is to provide a steam turbine with an improved cooling system for the casing.
The invention provides a steam turbine which includes a rotor which extends along a longitudinal axis and has at least two different pressure turbine portions disposed around and along the rotor. Each of the turbine portions has multiple stages. A casing encloses the rotor and has at least one steam inlet. A dummy ring is provided stationarily around the rotor to separate and seal between the two different pressure turbine portions. The dummy ring defines a nozzle chamber for receiving the steam from the steam inlet port and a plurality of nozzles for directing the steam from the nozzle chamber toward the higher pressure turbine portion. The dummy ring and the casing define a space therebetween. The space is fluidly connected to the higher pressure turbine portion. Piping extends between the space and a steam passage downstream of the last stage of the higher pressure turbine portion. The steam passage allows the steam within the space to flow to the steam passage downstream of the last stage of the higher pressure turbine portion to cool the casing.